At the present time, gas turbine engines are employed in a wide variety of environments, including on board aircraft. These engines typically include turbocompressors having a compressor wheel with a plurality of spaced apart blades on that wheel. The compressor wheel is rotated about an axis within the engine housing to receive air from an inlet, accelerate and compress that air, and then discharge the air through an outlet. To be most efficient, the air is generally forced to flow between the space defined by the blades, the rotational hub of the compressor wheel and a portion of the engine housing commonly referred to as a compressor shroud. That shroud is normally positioned adjacent the compressor blades opposite the hub.
Compressor efficiency is often greatest when a minimal running clearance is maintained between the shroud and the blades to prevent leakage of the air over the top of the blades. However, during normal operation of the compressor, centrifugal forces acting on the compressor wheel cause it to "grow" radially in the direction of the shroud. Thus, establishing minimum running clearance at operational speeds of the compressor can be a complex task given the variables involved. An error in shroud position could result in a significant loss of operating efficiency or cause damage to the compressor when the blades bind against the shroud.
In prior compressors it has been suggested to employ abradable coatings on the shroud to directly establish the minimum running clearance. Specifically, on the surface of the shroud adjacent the blades a coating is formed from a material which can be readily abraded by the edges of the compressor blades. Initially, a small clearance, on the order of 0.010 inches, is established between the blades and the coating when the compressor is in a quiescent state. Upon operation of the compressor, the centrifugal forces may, for example, cause radial "growth" of the compressor on the order of 0.010-0.015 inches. As a result of that growth, the blades will contact the coating and wear away part of the coating to automatically establish the minimum running clearance during compressor operation. This clearance is substantially less than the clearance which exists when the compressor is not operating.
A number of factors must be considered in selecting an appropriate material for use as an abradable coating on a compressor shroud, depending at least in part upon the composition, use and operating environment of the compressor. For example, the coating should ideally be abradable without damage to the blades, compressor wheel or shroud, and the material removed also should not affect any components downstream from the compressor, such as the combustor or turbine in a gas turbine engine. With respect to use in line with a combustor, the effect of having the removed material pass through the combustion process should also be considered.
At the same time, it is desirable for the abradable material which is not intended to be removed by the blades to remain securely bonded to the shroud both while other portions of the coating are removed by the blades and afterwards for the operational life of the compressor. As an illustration, in gas turbine engines of aircraft, the compressor and shroud are subject to wide ranging temperatures during each operational cycle, from, for example, -65.degree. F. (when quiescent at high altitude) to over 500.degree. F. (when operating). Moreover, it is expected that these engines are to be relatively maintenance free for a large number of operational cycles. Therefore, an ideal abradable coating would remain secure despite repeated thermal cycling of compressor start up and shut down. The same would be true for compressors used in other environments and for other purposes since air compression itself typically causes elevated temperatures in multi-stage compressors and often more so in single stage compressors.
In aircraft applications it is also important that coating weight be minimized and that repair and/or refurbishing of the coating be relatively easy so that it is not necessary to replace the more expensive engine components as well. Moreover, the gas turbine engines found in auxiliary power units of aircraft often have complex shroud configurations. Accordingly, any material selected for such a coating would ideally be relatively easy to apply to such shapes.
Previously, a variety of abradable coatings have been suggested for use in compressor shrouds and related applications. By way of illustration, attention is directed to U.S. Pat. Nos. 3,346,175 to Wiles; 3,547,455 to Daunt; 3,843,278 to Torell; 4,460,185 to Grundey; and 4,666,371 to Alderson.
For compressor shrouds formed from metal, abradable coatings have been formed from metal alloy powder suspended in a polyester resin. However, since polyester resin often is effectively limited to temperatures under 350.degree. F., such coatings would be similarly limited in their applications. To overcome that limitation and increase the temperature range to 550.degree. F., it has been suggested to apply polyester-based coatings to the shroud by a process such as flame-spraying. It appears that this increase in operating range is available because the polyester component of the resin is largely pyrolized during the flame-spraying process. Thus, the metal powder becomes bonded to the metal of the shroud by the resin residue and intermetallic bonding.
However, the flame-spraying process is relatively complex, especially with complex shroud shapes and has not been entirely successful in providing acceptable coatings on shrouds or other substrates that are formed from composite materials. To improve fuel efficiency by weight reduction, improve long term operating performance and to decrease production costs it has been suggested that composite materials be employed, for example, in aircraft auxiliary power units. Such composites are often formed of a fibrous mass supported in a thermoset matrix material or resin.
Composite structures differ significantly from those of metal. In general, composites are characterized by a higher degree of stiffness and lower density. Composite parts using fibers of carbon, graphite or boron, in particular, can have higher damping ratios than corresponding metal parts. Also, composites are more resistant to heat and abrasion. Further, composites often have a relatively low coefficient of thermal expansion (CTE). As an illustration, it has been found that for composite matrices using carbon or graphite fibers and operating in a gas turbine engine environment the CTE is approximately 2.0.times.10.sup.-6 in./in. .degree.F.
Unfortunately, it has been found that the abradable coatings used on metal shrouds are not sufficiently durable on composite shrouds because they tend to delaminate or otherwise separate from the shroud as a result of the thermal cycling. It is now believed by the applicants that this delamination is at least in part a result of significant differences in the CTEs of the shroud and the coating. For exemplary comparison with the above-noted CTE, it has been found that the CTE of polyester/metal coatings is approximately 14.times.10.sup.-6 in./in. .degree.F.
In other environments and applications, different compositions of abradable coatings have previously been suggested. Some of these have provided a light weight, low density coating by suspending hollow particulate matter, referred to as "microballoons", in a matrix material. Specifically, it has been suggested to use an abradable coating of small phenolic spheres in an epoxy or rubber matrix. Unfortunately, such coatings have limited applicability with certain products, such as gas turbine engines for use as auxiliary power units in aircraft, because epoxy breaks down at temperatures in excess of 350.degree. F. and epoxy and phenolic spheres have a relatively high CTE, on the order of 14.0.times.10.sup.-6 in./in. .degree.F. and 28.0.times.10.sup.-6 in./in. F., respectively. Thus, such coatings would be subject to an undesirable level of delamination as a result of thermal cycling of the engine.
Accordingly, it is an object of the present invention to provide an improved abradable coating. Other objects include the provision of an abradable coating which is:
1. of low density and weight, PA1 2. inexpensive to produce and relatively easy to apply, repair and renew, PA1 3. chemically inert within the operating environment and sufficiently tough to resist cracking, PA1 4. resistant to delamination as a result of thermal cycling over a wide range of temperatures, PA1 5. durable at operating temperatures in excess of 500.degree. F., and PA1 6. well suited for application to compressor shrouds formed from composite materials and used in aircraft auxiliary power units.
Further objects include the provision of new and improved gas turbine engines and aircraft having auxiliary power units through the use of durable and inexpensive abradable coatings on composite components therein.